Since this was a headache for me I thought I'd put a quick tut together for others…
I assume your freenas box is working and you can access it from your pc. You have tomato installed on your router and the image supports cifs (Administration/CifsClient can be found in the web gui).

Note that the recommendation is to use a known userid password on your nas box rather than guest access or anonymous in order to get your router's cifs client to connect properly. So that is how we will set this up below.

services/cifsAndSmb/shares(tab)
take note of the name you want to mount, in my case you will see below it is "data"
path=/mnt/nas/ name=data comment=blah browsable=yes
<assume you already have something set here, if not click + and add your drive mount(s)>

services/cifsAndSmb/settings(tab)
(note checkbox is in the title bar) Common Internet File System: checked <hmm, guess that's what CIFS stands for>
auth Anonymous
netbios mynas
workgroup MYHOME
(rest of settings on this page don't matter for our purposes)
<assume you already have set up here and are just noting the field settings, if you made changes: click save&restart>

network/lan/ipv4(section)
type static
ipAddress 192.168.1.99/24
gateway 192.168.1.1
<static is the way to go IMHO (or use staticDHCP on your router), address must be outside range of lan dhcp addresses, if you made changes: click save>

Next on your tomato web gui:

advanced/firewall
natloopback=all
(believe/guessing that the above is only necessary if you are accessing freenas that is on the wan port side of your router, in my test case i have an asus router's wan port plugged into my primary linksys router's lan port, the freenas box is on the linksys lan as well)
<save>

administration/cifsClient
/cifs1
enable checked
unc \\192.168.1.99\data
(note that "data" from freenas/services/cifsAndSmb/shares(tab) used above. do not use \\192.168.1.99\mnt\nas)
netbiosName mynas
user xyz
pass xyz
domain MYHOME
execute (leave blank)
security none
<save>
(if after saving it comes back with total/freeSize line showing something sensible, then it worked, if it shows "mounting…" forever, you did something wrong likely)